This patent specification is in the field of magnetic resonance imaging (MRI) and is directed to improvements including techniques particularly important in high field MRI.
The 1999 patent application referenced above describes simultaneous image refocusing (SIR) that includes giving each of several MRI slices a respective phase history different from that for other slices, for example by pulses on the read gradient (Gr) axis interspersed with the RF excitation pulses, and simultaneously refocusing all the slices several times in the same MRI signal acquisition sequence.
At higher MRI field strengths the RF penetration of the human body is markedly reduced. To compensate for this, the power of the RF pulses can be increased, but this in turn leads to an increased heating of the subject. Rapid spin echo (SE) sequences, also called turbo spin echo (TSE), fast spin echo (FSE), and rapid acquisition with relaxation enhancement (RARE), that use a large number of closely spaced high flip angle RF pulses, have to be slowed down considerably to prevent harming the patient. Currently offered 3T MRI scanners are examples of such issues, and higher fields would show an even greater reduction in RF penetration.
SIR-based TSE sequences help deal with the reduction of RF penetration with an increase of MRI field strength from two directions. First, their longer readout periods increase the spacing of the RF refocusing pulses. Second, the number of refocusing pulses they apply is significantly reduced because each RF pulse acts on several slices simultaneously. Therefore, the amount of deposited RF energy per unit time that has to be dissipated by the subject's body is much lower in SIR-based sequences, and sequences such as SIR TSE can be run faster or with larger slice coverage.
In SIR TSE sequences some of the stimulated echoes from the first slice may not all fall on top of the primary echo from the same slice but instead may interfere with the primary echo from the second slice (In an n-slice 3 slice SIR TSE sequence). One way to avoid such interference would be to spoil all stimulated echo pathways. This can be done by constantly increasing the spoiling pulse amplitude. This can suppress all the stimulated echo pathways efficiently but is usually limited by the maximum gradient strength of the scanner hardware. Another disadvantage of this approach is that all echo pathways are suppressed, including those that fall on the correct side of the readout and could contribute to the signal without interference. The approach disclosed in this patent specification removes both disadvantages—it suppresses the stimulated echoes that would interfere with the primary echoes from other slices while allowing the contribution of the non-interfering stimulated echoes, but does not require increases in gradient strength. The new approach thereby provides MRI benefits in addition to those from SIR alone.
Let us call the stimulated echoes of one slice, which fall on top of the primary echo of another other slice, the “wrong stimulated echo,” and call the stimulated echoes that contribute to the primary echo of the same slice, the “right stimulated echoes”. Wrong stimulated echoes result from echo pathways that miss an odd number of refocusing pulses. Since in SIR TSE sequences the position of the primary echo within the readout period changes every refocusing period, those wrong stimulated echoes may fall on top of the primary echo of another other slice and cause interference. Right stimulated echoes, on the other hand, miss an even number of refocusing pulses and fall on the correct side of the readout period. Since the position of the primary echo of one slice changes with a period of 2, the spoiling scheme should have the same period of 2. If the suitable gradient spoiling is chosen as a unit measure, a suitable spoiling scheme would be, e.g.:                (2,1,2,1,2,1,2,1, . . . ), or        (1,2,1,2,1,2,1,2, . . . ), or        (−1,1,−1,1,−1,1, . . . ), or        (2,3,2,3,2,3,2,3, . . . ),where successive numbers give the relative gradient momentum for successive refocusing periods.        
The constraints are:                periodicity of 2        difference of momentums of spoiler pulses between refocusing periods at least one unit measure        total rephrasing of primary echo during readout period        
Such encoding of multiple spin echoes within the RF pulse train is a departure from conventional CPMG conditions that could raise concerns. However, this patent specification describes a novel periodic spoiler pulse scheme that selectively removes only the stimulated echoes that would interfere with the spin echo of an alternate slice but does not spoil the coherent stimulated echoes. The new method alternates gradient amplitudes between odd and even RF refocusing periods. This is different from earlier described methods of progressively increasing the amplitude of the spoiler pulses in non-CPMG sequences, which removes all of the stimulated echo magnetization. This is also different from Alsop's method (see reference 1) of RF spoiling half of the total signal in a non-CPMG sequence. The clinical application for SIR TSE described in this patent specification is suitable for T1-weighted imaging that uses fewer RF refocusings generating much less stimulated echo signal with 3 to 5 RF refocusings as compared to T2 imaging that typically uses more than 5 RF refocusings. An advantage of T1-weighted imaging is using centric k-space ordering that places the center of k-space on the first RF refocusing period, which does not have stimulated echoes. Using this scheme, the new approach described here can produce very high quality T1-weighted SIR TSE images of the brain and body, using no signal averaging. Phase cycling in two averages, removes the potential high frequency signal leakage from images. Examples of comparing two average SIR images with and without phase cycling found no detectable difference between the two sets of images, indicate freedom from artifacts from the overlap of high spatial frequency k-space. Preliminary findings in many different image acquisitions of different matrix sizes and sagittal, coronal and axial planes through the head and in phantoms indicate that without signal averaging or phase cycling, SIR TSE images are free of artifacts, appearing the same as conventional TSE images. Nevertheless, when using two or more signal averages, phase cycling removes spurious signal or artifact from high spatial frequency overlap or stimulated echoes without the need for spoiler pulses.
At 3T imaging where the signal-to-noise ratio (SNR) is expected to be 100% higher, the SIR TSE is not dependent on two averages, and therefore can be performed with large reduction in SAR (specific absorption ratio, which is a measure of RF heating of the subject), and increased slice coverage with the time-savings of a single-average acquisition. One proposal for a different approach to reducing SAR in TSE is discussed in Hennig and Scheffler (see reference 2).
The novel alternating amplitude spoiler scheme described here reduces signal amplitude progressively through the echo train. Examples of phantom and head measurements have indicated that the first echo data has 100% signal compared to conventional TSE, the second echo has 99%, and the third echo 97% for certain images. The amount of net stimulated echo spoiling is dependent on the RF excitation profiles, and is believed improvable with improvements in the 180° pulse slice profiles, which have not been optimized for SIR.